Those who use wheelchairs, patients who are bedridden, airline pilots, truck drivers and others who must remain sitting or lying in one position for long periods of time typically face the prospect of developing decubitus ulcers or "bed sores." These ulcers are believed to originate in part from pressure and a lack of circulation in areas of the body supporting the person. Such ulcers are painful and they present the risk of large-scale serious infection which often penetrates deeply into tissue within the body. As is the case in management of many other types of medical problems, frequently the best treatment for such a condition is prevention. It has recently been found that total relief from pressure on an area of the body from time to time is required in order to minimize the possibility of decubitus ulcer formation in that area.
Previous approaches to preventing decubitus ulcer formation include uses of water-filled cushions, foam cushions and cushions inflated with air. Air-filled cushions enjoy a number of advantages in such applications, including weight, expense and ease of controlling cushion pressure.
Many earlier approaches recognize the value of a cushion which has two sets of pockets or cells which may be alternately inflated and deflated to remove pressure from the patient's skin. U.S. Pat. Nos. 4,225,989 issued Oct. 7, 980 to Corbett et al.; 4,175,297 issued Nov. 27, 1979 to Robbins et al.; 3,674,019 issued July 4, 1972 to Grant; and 3,587,568 issued June 28, 1971 to Thomas all disclose mattresses or pillows having two or more sets of elongated pockets or cells. The Corbett, Robbins and Thomas patents show mattresses having two sets of elongated pockets in which a valve alternately inflates and deflates the pockets. The Corbett and Thomas patents utilize fluid valves or switches to divert pressurized air alternately from one set of pockets to the other. Control of pressure at which the cushion is inflated and the time intervals for inflation are perhaps best controlled by controlling air pressure applied to the fluid switch. Such control is imprecise, however, and control of inflation pressure is linked to inflation interval.
The Robbins patent discloses an electrically controlled valve which controls inflation of two sets of pockets in a pillow. Air pressure applied to the pockets may be adjusted by adjusting set screws in the valve. Although pressure may perhaps be regulated independently of inflation sequence according to this arrangement, pressure regulation by turning set screws can be cumbersome and inconvenient, especially for a bedridden or immobile person.
U.S. Pat. Nos. 4,617,690 issued Oct. 21, 1986 to Grebe and 4,551,874 issued Nov. 12, 1986 to Matsumura et al. disclose the concept of controlling inflation sequence in a number of sets of pockets or cells of a cushion in order to minimize onset of decubitus ulcers. These patents thus address several shortcomings of the cushions which only alternately inflate two sets of pockets or cells. First, they eliminate the tendency of such cushions merely to recompress a skin area which has just been relieved of pressure as pockets or cells are alternately inflated and deflated. Second, inflation cycle and pressure can be adjusted to relieve pressure on portions of the patient under most pressure and thus most prone for development of ulcers, such as areas supported by bony regions. Matsumura, for instance, discloses an inflatable bed mattress with pockets whose inflation is controlled by a number of three-way valves which may be manually controllable or automatically controllable.
Recent advances in cushion technology have made available cushions with a number of independently inflatable cells, rather than elongated pockets as disclosed in the patents mentioned above. Cushions formed of these independent cells can more readily and easily conform in shape and pressure to the persons they support. U.S. Pat. No. 4,422,194 issued Dec. 27, 1983 to Viesturs et al., which is incorporated herein by this reference, discloses such a cushion. U.S. Pat. Nos. 3,870,450 issued Mar. 11, 1975; 4,005,236 issued Jan. 25, 1977; 4,541,136 issued Dec. 17, 1985; and 4,698,864 issued Oct. 13, 1987 to Graebe, which patents are incorporated herein by this reference, also disclose such cushions. U.S. Pat. 4,005,236 to Graebe, for instance, discloses a multi-cell structure which has a number of rows and columns of interconnected inflatable cells. U.S. Pat. No. 4,698,864 to Graebe discloses such a cushion in which the cells are arranged in zones with the interiors of the cells for each zone being in communication and isolated from the interiors of cells in other zones. The cells may be of varying height to contour the surface of the cushion for additional comfort. These patents for cellular cushions fail to disclose a system for controlling sequence, interval and pressure of cell inflation, however.